Gravimetric diluter

ABSTRACT

An apparatus for making dilutions of a sample with a diluent to a predetermined dilution factor f by weight of the diluent is disclosed. A scale with TARE capability produces an output signal of the weight W of a sample within a container less the weight of the container. A dilution factor control is provided for setting the predetermined dilution factor f. Apparatus is provided for calculating and storing the value TW=W/f which is the total weight of the sample plus the weight of the diluent which is required to dilute the sample to the predetermined dilution factor f. A pump is connected between a reservoir of diluent and the container for the sample for pumping the amount of diluent into the container which is required to dilute the sample to the predetermined weight dilution factor f. The scale provides a continuous output of the weight W of the sample plus diluent in the container at any instant in time. Apparatus is provided for calculating when TW-W=0. In one embodiment, the flow of diluent to the container is stopped by the stopping of the pump when TW-W=0. In another embodiment, the flow of diluent to the container is stopped by closing a pair of valves located between the pump and the container when TW-W=0.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to application Ser. No. 232,531, entitled GravimetricDiluter, which was filed on the same date as the present application.That application, which is assigned to the assignee of the presentapplication and names Samuel Schalkowsky and Donald Whitley asinventors, discloses and claims a gravimetric diluter which utilizes apump to control the flow of diluent into a container to make dilutionsby weight to a dilution factor f.

BACKGROUND OF THE INVENTION

The invention relates to apparatus for making dilutions of samples ofany weight with a diluent to a predetermined weight percentage ofdiluent.

DESCRIPTION OF THE PRIOR ART

In the fields of microbiology, biochemistry and chemistry it is oftennecessary to dilute liquid or solid samples with a diluent to apredetermined concentration by weight. In the case of solid samples, theliquid diluent may be used to make a slurry of the solid in the liquid.Bacterial density in solid food or fruit juice samples is determined byanalysis of dilutions of these samples. In certain areas such asclinical laboratories or quality control laboratories it is necessary tomake dilutions of large numbers of samples. While it is possible tomanually dilute samples with diluents to a predetermined percentage byweight or volume, the amount of time required can be great where highaccuracy or large numbers of dilutions are required.

Automatic counting and packaging systems are known. In these systems thenumber of items which are packaged is determined by entering the totalweight of the package of items into a microprocessor control anddividing that quantity by the average weight per item to determine thenumber of items.

Systems are known which make weight measurements of individualingredients for making concrete or other substances which require aprecise weight ratio of the ingredients to make a satisfactory compositemix of the ingredients. These systems are programmable to permit anoperator to make a predetermined mixture of, for example, 100 pounds ofingredient A, 200 pounds of ingredient B and 300 pounds of ingredient C.Unlike the present invention, these systems do not make automaticdilutions of a first ingredient of unmeasured weight to a predeterminedpercentage by weight of a second ingredient.

Pumping systems are known which simultaneously control the pumping oftwo or more liquids in a predetermined volumetric ratio. Examples of useof such systems are in the field of chromatography, the mixing of twogrades of hydrocarbons such as gasoline or oil and in the field ofchemical processing. These systems do not make a dilution of a sample ofan unknown weight with a diluent to achieve a predetermined weightpercentage of diluent in the sample.

SUMMARY OF THE INVENTION

The invention is an apparatus for diluting liquid or solid samples to apredetermined concentration by weight of a diluent. The invention hasmeans for measuring the weight W of a sample within a container less theweight of the container; means for specifying a predetermined dilutionfactor f by weight of a diluent in a sample; means for calculatingTW=W/f; means calculating TW-W, fluid conducting means disposed betweena reservoir of diluent and the container for permitting the diluent toflow from the reservoir to the container for the purpose of diluting asample within the container to the predetermined concentration by weightof diluent f; and selectively closeable valve means in fluidcommunication with the fluid conducting means which is disposed betweenthe reservoir and the container for interrupting the flow of diluentbetween the reservoir and the container, said valve means being coupledto the means for calculating TW-W and being closed when TW-W=0 and openwhen TW>W. The valve means is pulsed on and off when TW-W<1 to preventovershoot in the amount of diluent added to the sample for any value off. The invention also includes means for specifying a maximum weightlimit UL of the combined weight of sample and diluent which isacceptable for a given dilution factor f and means for specifying aminimum weight limit LL of the combined weight of sample and diluentwhich is acceptable for a given dilution factor f.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system schematic of one embodiment of the present invention.

FIG. 2 is a detailed schematic of a control for the embodiment of thepresent invention illustrated in FIG. 1.

FIG. 3 is a flow chart of the preferred form of a microprocessor controlfor the embodiment of FIG. 1.

FIG. 4 is a system schematic of another embodiment of the presentinvention.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a general system schematic of the present invention 10 whichfunctions to automatically dilute a sample 11 contained within acontainer 20 to a predetermined weight dilution factor f of a diluent36. In accordance with the invention, dilutions of samples 11 are madewithout requiring measurement of the sample weight prior to placement inthe container 20. The dilutions made by the present invention are byweight percentage of the diluent in the sample 11. The dilution factor fof the present invention is defined by the equation: ##EQU1## Forexample, a 100 gram solution of ethanol which has been diluted with 100grams of water is diluted to a weight dilution factor f=0.5 which equalsa 50% dilution by weight of diluent. In accordance with the invention, ascale 12 with a TARE control 14 is provided which has an output line 16on which is continually produced a signal of the weight placed on thepan 18 in binary coded decimal (BCD) format. In accordance with theinvention, the TARE control 14 is activated to produce a zero weight BCDoutput signal on line 16 when the pan 18 has a container 20 resting onit within which a sample 11 will be placed which is to be diluted bydiluent 36. Scales are commercially available with TARE controls whichproduce a BCD output which may be used in the present invention. Asuitable commercially available scale having TARE capability and a BCDoutput which may be used with the present invention is a model PS15which is manufactured by the Mettler Instrument Corporation ofHightstown, N.J.

The controller 22 of the present invention includes a plurality ofcontrols which include a dilution factor selector control 24, anautomatic dispenser switch 26, a manual reset switch 28, an "on"-"off"power switch 30, a rapid manual dispenser switch 32 which is used tomanually open a first rapid dispense valve 34 to the flow of diluent 36from reservoir 38 through conduit 40, under the power of pump 42, a slowdispense switch 44 which is used to manually open a second valve 46 tothe flow of diluent 36 from reservoir 38 through conduit 40 under thepower of pump 42, a ready light 46A, an excessive sample indicator light48, an insufficient sample indicator light 50, a power on indicatorlight 52, maximum weight control 53 and minimum weight control 55. Themaximum weight control 53 specifies the maximum weight of sample plusdiluent which is permissible for the dilution of a sample to a specifieddilution factor f. The minimum weight control 55 specifies the minimumweight of sample plus diluent which is permissible for the dilution of asample to a specified dilution factor f. The rapid manual dispenseswitch 32 and the slow manual dispense switch 44 may be used to manuallyoverride the automatic control of valves 34 and 46 by the controller 22to permit selective flow control of diluent into container 20. The firstvalve 34 has a rated flow rate for a given pressure which is 10 timesgreater than the flow rate of the second valve 46. The controller 22 hasan interlock line 54 which is coupled to a switch 54A and attached tothe chassis of the scale 12 which allows valves 34 and 46 attached tothe horizontal member 76 to operate when the stanchion 74 is rotated sothat switch 54A is closed and valve 34 and valve 46 are over thecontainer 20, a power line 56 which provides suitable electrical powerfor the scale 12 and a pump activation line 58 which controls theactivation of pump 42 for pumping diluent 36 from reservoir 38 throughconduit 40, through either the first valve 34 or the second valve 46into container 20 in a manner to be hereinafter described. The powerline for the controller 22 has been omitted. The conduit 40 contains a Tsection having a input which is coupled to the pump 42 and first andsecond outputs 60 and 62 which are respectively connected to the firstand second valves 34 and 46. Valve control line 64 is connected betweenthe controller 22 and the first valve 34 for selectively controlling theflow of diluent 36 from the pump 42 through the first valve in a mannerto be hereinafter explained in conjunction with FIGS. 2 and 3. Valvecontrol line 66 is connected between the controller 22 and the secondvalve 46 for selectively controlling the flow of diluent 36 from thepump 42 through the second valve 46 in a manner to be hereinafterexplained in conjunction with FIGS. 2 and 3. The valves 34 and 46 mayhave solenoids which are activated under the control of activationsignals from the controller 22 over control lines 64 and 66. Valves 34and 46 may be model V5 valves sold by Skinner Precision Industries, Inc.of New Britain, Conn. Bacterial filters 68 may be coupled between theoutputs 70 and 72 of the first and second valves 34 and 46 to filter thediluent 36 prior to discharge into container 20. The valves 34 and 46are supported above container 20 by a metallic stanchion 74 and ahorizontal member 76.

FIG. 2 illustrates a detailed electrical schematic of an embodiment ofthe invention. It should be understood that the embodiment of FIG. 2uses digital storage and computation techniques. However, it should alsobe understood that the present invention may be implemented with equalfacility as an analog system. As described in conjunction with FIG. 1,the scale 12 produces a BCD output of the weight on the pan 18 on line80. The TARE control 14 is activated after the container 20 is placed onthe scale 12 but prior to the introduction of a sample 11 which is to bediluted to a predetermined dilution factor f specified by the dilutionfactor selector control 24. The automatic dispense switch 26, which isdepressed after the TARE control 14 is activated and the sample 11 to bediluted with the previously set dilution factor f has been placed in thecontainer 20, causes scale 12 to start to produce a BCD output of theweight of the sample plus diluent in the container 20 on line 16. TheBCD output from scale 12 on line 16 is coupled to a sample circuit 82 ofany conventional design that samples the BCD output of the weight of thesample 11 prior to the introduction of any diluent. The sample circuit82 only couples the BCD output from the scale 12 to the means forcalculating 86 when the signal from delay 84 is high. The time at whichthe BCD output is sampled by sample circuit 82 is controlled by thedelay 84 which may be a one shot multivibrator or other suitableelectrical delay, that produces a high level output signal for a shorttime interval after the automatic dispense switch is depressed. The timedelay produced by the delay 84 need only be sufficient to permit the BCDoutput of the sample 11 to appear on line 16 after the depressing of theautomatic dispense switch 26. The output of the sample circuit 82, iscoupled to the means for calculating 86 which calculates TW=W/f where Wequals the weight of the sample to be diluted and f equals the weightdilution factor f specified by the dilution factor selector control 24.Since the sample circuit 82 only couples the BCD weight of the sample 11in the container 20 to be diluted to the means for calculating 86, for ashort time interval after depressing the automatic dispense switch 26during which the output from delay 84 sample circuit is high, the weightrepresented by the calculated weight TW is equal to the combined weightof the sample and diluent which will be present in the container 20 whenthe sample has been diluted by the diluent by the weight dilution factorf. The dilution factor selector control 24 may be a combination of apotentiometer, amplifier and analog to digital converter. The circuitryrequired to convert the analog input f varying such that 0<f<0.999 intoa digital format is conventional and forms no part of the presentinvention. The output TW from the means for calculating 86 is stored ina memory 88 of conventional design which is resettable by the manualreset switch 28. The depressing of the automatic dispense switch 26 alsocauses digital output signals to be produced from the maximum weightlimit control 90 and the minimum weight limit control 92 after a timedelay produced by delay circuit 94. The maximum and minimum weightlimits, UL and LL of sample plus diluent, are set before depressing ofthe automatic dispense switch 26. The setting of the maximum and minimumweight limits may be by means of DIP switches or other suitable analogor digital inputs. In the preferred embodiment, the maximum and minimumweight limits may vary between 0<LL, UL<9999 grams. The time delaycircuit 94 may be a one shot multivibrator of the same type as delay 84.The delay produced by delay 94 is longer than that produced by delay 84in order to insure that the calculated weight TW has been calculated andstored prior to the output of the maximum and minimum weights fromcontrols 90 and 92 respectively. The output from the maximum weightcontrol 90 is coupled to a means for calculating 96 if TW-UL<0. Theoutput from the minimum weight control 92 is coupled to a means forcalculating 98 if TW-LL<0. The output from the means for calculating 96and 98 are coupled to the stop input of the pump 42 which immediatelydisables the pump if the calculated weight TW is greater than thespecified maximum weight UL from the maximum weight limit control 90 orthe specified minimum weight LL from the minimum weight limit control92. If either condition is present, the system is shut down and anappropriate excessive sample indicator light 48 or an insufficientsample indicator light 50 is lit. If the calculated weight TW does notfall outside the specified maximum and minimum weight limits, theactivation of the pump after the expiration of an additional delayproduced by delay 104 occurs. The additional delay 104 insures that thepump does not start after the depression of the automatic dispenseswitch 26 for the case where the calculated weight TW for a dilutionwould fall outside the maximum and minimum weight limits specified bycontrols 90 and 92. In the meantime, the scale 12 starts to continuallyproduce a BCD output of the weight of sample 11 plus diluent 36 in thecontainer 20. The BCD output is coupled to a means for calculating 106the quantity TW-W. The means for calculating 106 determines if thequantity TW-W is greater than 10. If TW-W is greater than 10, a controlsignal is applied by the means for calculating 106 to the first valve 34to cause diluent to flow only through valve 34 at the highest flow rate.The means for calculating 106 also determines if the quantity TW-W isgreater than one but equal to or less than 10. If 1<TW-W<10, a controlsignal is applied by the means for calculating 106 to the second valve46 to permit diluent to flow from the reservoir 38 through the pump 42,conduit 40 and the second valve 46 to the container 20. When the controlsignal is applied to the second valve 46 to only permit diluent to flowthrough the second valve, the control signal is removed from the firstvalve 34 to cause it to close. The relative rated flow rates of thefirst valve 34 and the second valve 46 permit rapid filling of thecontainer 20 to a combined weight of sample plus diluent to within 10grams of the quantity TW and thereafter slower filling until TW-W=1.When the means for calculating 106 determines that TW-W=1, a pulsatingsignal is applied to valve 46 to turn on the valve for 0.2 seconds andthen to turn off the valve for 1.8 seconds. The pulsating control signalis repeatedly applied to valve until TW-W=0. The purpose of pulsatingthe valve 46 "on" and "off" when 0<TW-W<1 is to permit the inherent lagin the production of the BCD output signal from the scale 12 on line 16to not be a factor in shutting off the flow of diluent when TW-W=0.Otherwise overshoot in the amount of diluent added to the sample couldoccur. When TW-W equals zero, the means for calculating 106 produces anoutput signal which closes the second valve 46 to prevent further fluidflow through the second valve into the container 20 and shuts off thepump 42. At this instant in time, the dilution of the sample has beencompleted to a predetermined weight dilution factor f.

FIG. 3 illustrates a flow chart of the preferred form of implementationof the controller 22 for the invention which utilizes a microprocessorto control the system operation. At the starting point 200 of themicroprocessor control program, it is assumed the following sequence ofevents has occurred: maximum and minimum weights UL and LL of samplesand diluent have been entered; the dilution factor f has been set; thepower has been turned on; a sample container 20 has been placed on thepan 18; the TARE control 14 has been depressed, a sample 11 has beenplaced in the container 20; and the automatic dispense switch 26 hasbeen depressed. The program proceeds to block 202 where the ready lighton the controller is turned off to signify entry into a dilution of asample. The program proceeds to block 204 where the weight W of thesample to 0.1 g in BCD format is read from the scale 12. The programproceeds to block 206 where the weight W is stored in memory. Theprogram then proceeds to block 208 where the dilution factor f is readfrom the dilution input factor control 24 within the controller 22. Theprogram then proceeds to block 210 where the dilution factor f is storedin memory. The program then proceeds to block 212 where the maximumweight limit UL of the sample and diluent is read from the maximumweight control 53 in the controller 22. The range of the maximum weightlimit is 0<UL<9999 grams. The program then proceeds to block 214 wherethe maximum weight UL is stored in memory. The program then proceeds toblock 216 where the minimum weight limit LL of sample and diluent isread from the minimum weight control 55 in the controller 22. The rangeof the minimum weight limit is 0<LL<9999 grams. The program proceeds toblock 218 where the minimum weight LL is stored in memory. The programthen proceeds to block 220 where the calculated weight TW=W/f iscalculated and rounded to the nearest 0.1 g. The program then proceedsto block 222 where TW is stored in memory. The program then proceeds tothe decision point 224 where a determination is made if TW>UL. If theanswer is "yes" the program branches to block 226 where an over weightlight 48 in the controller 22 is turned on to indicate that the weightof the sample plus the diluent for achieving the previously set dilutionfactor f is greater than the specified maximum weight which has been setwith the maximum weight control 53. The program then proceeds to block228 where the memory elements are reset with switch 28 in the controller22 to prepare the system for another dilution. The program then proceedsto stopping point 230 which terminates all activity in the program. Ifthe answer is "no" at decision point 224, the program proceeds todecision point 232 where a determination is made is TW<LL. If the answeris "yes" the program branches to block 234 where the insufficientindicator light 50 in the controller 22 is turned on to indicate thatthe weight of the sample plus the diluent for achieving the previouslyset dilution factor f is less than the specified minimum weight whichhas been set with the minimum weight control 55. The program thenproceeds to block 236 where the memory elements in the controller 22 arereset with switch 28 to prepare the system for another dilution. Theprogram then proceeds to stopping point 238 which terminates allactivity in the program. If the answer is "no" at decision point 232 theprogram proceeds to block 240 where the pump 42 is turned on and thevalve 34 opened. At this point, the pump is pumping diluent 36 into thecontainer at the higher rate of flow permitted by valve 34 than ispossible with valve 46. The program then proceeds to block 242 where theBCD output from scale 12 of the combined weight of diluent and sample isread. The program then proceeds to decision point 244 where adetermination is made if TW-W≦10. If the answer is "no" the programloops back to block 242 and to decision point 244 where the calculationTW-W≦10 is made. As long as TW-W>10, valve number 34 is open and thehighest flow rate of diluent into the container 20 takes place. Theprogram continues to loop until TW-W=10 at which point the programbranches to block 246 where valve 34 is closed and valve 46 is opened.At this point the flow rate into the container 20 has been reduced by afactor of 10 as a consequence of the lower rated flow capacity of valve46 in comparison with valve 34. The program then proceeds to block 248where the weight W of sample plus diluent is read from the scale 12. Theprogram then proceeds to decision point 250 where a determination ismade if TW-W≦1. If the answer is "no," the program loops back to block248. If the answer is "yes," the program proceeds to block 252 where thevalve is pulsed to be on for 0.2 seconds and pulsed off for 1.8 seconds.The program proceeds to block 254 where the weight of the sample plusdiluent is read from scale 12. The program then proceeds to decisionpoint 256 where a determination is made if TW=W. If the answer is "no,"the program loops back to block 252 where the valve 46 is again pulsed.The program continues to loop until TW=W at which time the programproceeds to block 258 where valve 46 is closed and valve 34 ismaintained in its previously closed state. At this point in time, thepredetermined weight dilution factor f of diluent has been achieved andthe combined weight of sample plus diluent equals TW. The program thenproceeds to block 260 where pump 42 is turned off, the ready light inthe controller 22 is lit and the memory elements of the controller arereset. The program then proceeds to stopping point 262 where no furtherprogram action occurs.

FIG. 4 is a general system schematic of the second embodiment 300 of thepresent invention. The second embodiment is identical to the firstembodiment of the present invention which is illustrated in FIG. 1 withthe exception that the apparatus for controlling the flow of diluent isdifferent. Identical parts in FIGS. 1 and 4 are identified by identicalreference numerals. In the second embodiment of the invention, the flowof diluent between the reservoir and the container is controlled by thestarting and stopping of a pump 302 without the first and second valves34 and 46 of FIG. 1. The pump may be any type of pump which stops thepumping of fluid immediately when a stop command is applied thereto onlines 304 and/or 306 from controller 22. There are many types of pumpswhich have this capability. The preferred type of a pump is aperistaltic pump which may be a model Master Flex Pump C-7500-00manufactured by the Cole-Parmer Corporation of Chicago, Ill. Theperistaltic pump is preferred because it may be controlled toimmediately cease the pumping of fluid. The present invention requiresthe immediate ceasing of fluid flow into container 20 when TW-W=0 inorder to achieve accurate dilutions which are very close to thepredetermined dilution factor f. In this embodiment, reciprocatingpumps, centrifugal pumps, rotary pumps, or other types of pumps whichhave a relatively high inertia during pumping would not be useful unlessthey contained valving which permitted the immediate stopping of theflow of liquid upon the application of the stop command from thecontroller 22. Without valving to immediately stop the flow of diluent,the inertia of the operating pump would result in the continued pumpingof diluent after the application of the stop command which would resultin a dilution to a greater dilution factor than the factor f.

The second embodiment may include a pump 302 which may be controlled topump at two different flow rates which are used such that the higherflow rate is used when the quantity TW-W>10 and the lower flow rate isused when 0>TW-W>10 in a manner analogous to the two valve mechanisms ofthe first embodiment.

A control system like the one which is illustrated in FIG. 2 and amicroprocessor flow chart like the one illustrated in FIG. 3 may be usedwith the second embodiment. The only difference between the controlsystem of FIG. 2 and the microprocessor flow chart of FIG. 3 for thefirst embodiment and the control system and microprocessor flow chart ofthe second embodiment is that in the second embodiment the valves 34 and46 are omitted and the signals for controlling the flow of diluent areapplied directly to the pump 302.

OPERATION

The operation of the invention to make a dilution of a sample of ethanolwith a 50% solution by weight (f=0.5) of water is summarized as follows.The maximum and minimum weights UL and LL have been previously set bythe maximum and minimum weight controls 53 and 55 of the controller 22and are assumed to not warrant system shutdown. The dilution factor f isset at 0.5 by the dilution factor selector control 24. The TARE control14 is activated to zero the BCD output of the scale 12 after thecontainer 20 has been placed on the pan 18. The sample of ethanol isthen placed in the container 20 and the automatic dispenser switch 26 isdepressed. The BCD output from the scale specifies that the sample is100 grams. The means for calculating TW: ##EQU2## reflects that thecombined weight of ethanol and water to achieve an 0.5 dilution factoris 200 grams. The pump 42 is immediately activated and rapid dispensevalve 34 is opened. The pump then proceeds to pump water into thecontainer 20 through valve 34 until the combined weight of the waterplus ethanol reaches 190 grams at which point the program causes rapiddispense valve 34 to close and slow dispense valve 46 is opened toreduce the rate of introduction of water into the container 20. When theweight of ethanol plus water W equals 199 grams, the valve 46 isrepeatedly pulsed on for 0.2 seconds and off for 1.8 seconds. The pump42 continues to pump water into the container through the pulsatingvalve 46 until the weight of the water plus ethanol equals 200 gramswhich is equal to the previously calculated TW. The pump 42 is then shutoff and valve 46 is closed to complete the dilution.

Although the present invention has been described with reference to twoparticular embodiments, it should be understood that those skilled inthe art may make many other modifications without departing from thespirit and scope of the invention as defined by the appended claims. Forexample, the invention is not limited to the particular disclosed valuesof the quantity TW-W which control the activation of valves 34 and 46 orthe pump 302. Moreover, the particular apparatus for calculating variousquantities does not form part of the invention since any well knowncalculating apparatus may be used.

What is claimed as new and desired to be secured by patent of the UnitedStates is:
 1. An apparatus for diluting a sample within a container to adilution factor f by weight of diluent comprising:(a) means forrepeatedly measuring the weight W of the sample plus any diluent withinthe container; (b) means for choosing a dilution factor f by weight of adiluent to be used for diluting the sample; (c) means for calculating TWwherein TW=W/f and W is the weight of the sample within the containerprior to the addition of diluent; (d) means for calculating TW-W; (e)fluid conducting means disposed between a reservoir of diluent and thecontainer for permitting the diluent to flow from the reservoir to thecontainer for the purpose of diluting the sample within the container tothe selected dilution factor f; and (f) valve means in fluidcommunication with the fluid conducting means and coupled to the meansfor calculating TW-W for interrupting the flow of diluent between thereservoir and the container when TW-W=0 and permitting diluent to flowbetween the reservoir and the container when TW>W.
 2. An apparatus fordiluting a sample within a container to a dilution factor f by weight ofdiluent comprising:(a) means for repeatedly measuring the weight W ofthe sample plus any diluent within the container; (b) means for choosinga dilution factor f by weight of a diluent to be used for diluting thesample; (c) means for calculating TW wherein TW=W/f and W is the weightof the sample within the container prior to the addition of diluent; (d)means comparing TW and W; (e) fluid conducting means disposed between areservoir of diluent and the container for permitting the diluent toflow from the reservoir to the container for the purpose of diluting thesample within the container to the selected dilution factor f; and (f)selectively closeable valve means in fluid communication with the fluidconducting means and coupled to the means for comparing TW and W forinterrupting the flow of diluent between the reservoir and the containerwhen TW=W and permitting diluent to flow between the reservoir and thecontainer when TW>W.
 3. The apparatus of claim 1 or 2 further comprisinga pump which is disposed between the reservoir of diluent and the valvemeans in fluid communication with the fluid conducting means for pumpingthe diluent from the reservoir to the container.
 4. The apparatus ofclaim 3 wherein the valve means comprises:(a) a first valve means havingan input and an output, the first valve means being selectivelycloseable to interrupt the flow of fluid between the input and theoutput, the input being coupled to the output of the pump and the outputbeing adapted to be coupled to the container; (b) a second valve meanshaving an input and an output, the second valve means being selectivelycloseable to interrupt the flow of fluid between the input and theoutput, the input being coupled to the output of the pump and the outputbeing adapted to be coupled to the container, the second valve meanspassing less fluid than the first valve means for any given pressure;(c) means for calculating when TW-W=C where C is a predetermined weight,and (d) means for maintaining the first valve means open and the secondvalve means closed when TW-W is greater than C and for maintaining thefirst valve means closed and the second valve means open when TW-W≦C andTW-W is greater than
 0. 5. The apparatus of claim 4 furthercomprising:(a) a T shaped fitting having an input and two outputs whichis coupled between the output of the pump and the input to the first andsecond valve means, the input of the fitting being coupled to the outputof the pump and the outputs of the fitting being coupled respectively tothe inputs of the first and second valve means; and (b) first and secondfilters coupled respectively to the outputs of the first and secondvalve means.
 6. The apparatus of claim 1 or 2 further comprising:(a)means for specifying a maximum weight of sample UL plus diluent; (b)means for specifying a minimum weight of sample plus diluent LL; (c)means for calculating if TW>UL; (d) means for calculating if TW<LL; (e)means for preventing the dilution of the sample with the diluent to thedilution factor f if TW>UL or TW>LL.
 7. The apparatus of claim 4 furthercomprising means for pulsing the second valve means on and off when thequantity TW-W≦D where D is less than C.
 8. The apparatus of claim 5wherein C is 10 grams and D is 1 gram.